1. Field of the Invention
The present invention relates to a magnetic toner for use in an image development apparatus such as a copier, printer, facsimile, and the like.
2. Description of the Prior Art
As a development method in an electrophotographic technique, there have been available conventionally a cascade development method, touchdown development method, jumping development method, and others. Among these methods, the cascade development as disclosed in the U.S. Pat. No. 3,105,770 has been known as a development method in which developer is sprinkled directly onto a photoconductor. The cascade development method was used for the first time in a practical copier for electrophotography.
Further, in the U.S. Pat. No. 3,866,574 disclosed is a development method in which mono-component toner is jumped onto a photoconductor by applying an AC bias voltage to a developing roller thereby to effect development. In this U.S. Patent, the AC bias voltage to be applied to the developing roller is used for the purpose of activating the movement of the toner, where it is described that the toner jumps to image areas and returns on the way to non-image areas on the photoconductor.
As an improvement of the AC bias application technique, there is disclosed a jumping development method in the Japanese Patent No. Publication 63-42256 (published in 1988). In this jumping development method, the toner is supported by a toner support member, and on the toner support member there is provided a doctor blade for regulation of a rigid body or elastic body at a minute distance from to the support member. The toner is formed into a thin layer by the doctor blade and transferred to a developing section, where the toner is deposited on the image areas of the photoconductor with the AC bias application. The technical concept of the Japanese Patent Publication 63-42256 differs from that of the U.S. Pat. No. 3,866,574 in that the toner is moved reciprocatingly at image areas and non-image areas.
As is known, in these development methods, the toners for use in electrostatic charge development are generally composed of resin components, coloring agents of pigments or dyes, and additive components such as plasticizers and charge control agents. As the resin components, natural resins or synthetic resins are used solely or by mixture as the case may be.
However, there has been desired a further improvement in quality of copied images and their long-term stability of image quality in recent years.
Moreover, nowadays, from a viewpoint of environmental protection, it is necessary to regulate unlimited disposal of industrial wastes. It is also important to recycle the wastes.
With the conventional constitutions and methods as described above, it has been well known to those skilled in the art that the cascade development is no good at reproduction of solid blacks. Another problem is involved in the cascade development in that the system for the development method would be large in scale and complex in structure. Further, the developing device as disclosed in the U.S. Pat. No. 3,866,574 has a drawback that high precision is required for the system, which leads to a complex structure and high cost. In the jumping development method, it has been indispensable to form an extremely uniform thin layer of a toner on a toner support member carrying a toner layer. In this method, another problem has been frequently involved in that there occurs a so-called sleeve ghost development in which there remains hysteresis of the preceding image of the toner thin layer formed on the toner support member, which causes an afterimage to appear in the resulting image. The method also has a problem that a complex equipment and high cost are required, disadvantageously.
Therefore, the applicant of the present invention has previously proposed a new electrophotographic system for development which is smaller in size and higher in performance without using a doctor blade. The development process in the new electrophotographic system is implemented by providing a photoconductor containing a stationary magnet, and an electrode roller having a magnet opposed to the photoconductor at a specified spacing. Thus, by this development method, it becomes possible to reproduce solid blacks with fidelity, free from occurrence of sleeve ghost, allowing the system to be further reduced in size, simplified in construction, and lowered in cost.
To improve the quality of images by using this development method, however, there is a need for even higher performance of toner properties. In this development method, since no doctor blade is provided for regulating the toner into a thin layer, the toner is transferred to the development field which is defined by a space of a narrow gap between the photoconductor and the electrode roller, without being formed into a layer. As a result, the place and space needed to obtain a desired charge amount for the toner to be triboelectrified is restricted to only a small one, so that the toner is required to have higher charging characteristic than in conventional one. It is the fluidity and electrical resistance of a surface of toner particles which affects the toner's high charging characteristic. Fluidity can be defined by apparent density, and the surface resistance can be defined by a dielectric loss.
The fluidity of the toners used in the conventional mono-component development or two-component development has drawbacks such that non-uniformities would occur in solid-black image areas and half-tone image areas and besides background fogs would increase in non-image areas. This phenomenon can be found remarkably in toners having low fluidity. This can be attributed to the fact that toners with low fluidity cannot afford satisfactory amounts of triboelectrification due to low possibility of contact with the developing member. Moreover, there arises non-uniformities in the triboelectrification performance among individual toners, so that uniform toner chargeability cannot be obtained. On the other hand, toners keeping high fluidity exhibit uniform contactability with the developing member, so that a high level of charge amount can be obtained, in which case high-quality images can be obtained.
For enhancement of fluidity of toners, there has been taken a measure of increasing an adding amount of additives such as silicon oxide, which is a fluidity imparting agent. However, if additives of such as silicon oxide are increased in quantity, the fluidity increases with an increasing amount of additives, while suspended particles of silicon oxide also increase in amount such that the silicon oxide is implanted as cores into the photoconductor by urging force of a cleaning blade, resulting in flaws. As a result, there would occur a phenomenon of filming that silicon oxide or toner adheres onto the photoconductor. Meanwhile, the suspended particles of silicon oxide adhere to solid black areas, bringing about white dots. In consequence, increasing the amount of silicon oxide results in disadvantages in many respects, offering no solution to the problems.
Conventional magnetic toners have magnetic materials internally added in the binder resin. The toner is pulverized to be finely divided. In this process, since the magnetic material that is lower in electric resistance than the binder resin is exposed on the toner surface, therefore the charges obtained through triboelectrification are likely to leak, such that the magnetic toner encounters difficulty in obtaining large amount of charges, to a disadvantage.
As a further aspect, earth environmental protection has come up to an issue of great significance in recent years. In conventional copiers, laser printers, laser facsimiles, and the like, a toner is developed on the photoconductor in the development process and then transferred onto paper in the transfer process. In these processes, a part of the toner remains on the photoconductor. The part of the remaining toner is swept down in the cleaning process. The cleaned-off toner results in residual toner. In the conventional methods, especially in the mono-component development, the residual toner is wasted, which can not be recycled.
In recycling the residual toner swept off the photoconductor in the cleaning process for once more development, the toners in the conventional method will result in non-uniform charge distribution if the residual toner is mixed with unused toner within the developing unit, such that wrong sign toner of reversed magnetic polarity increases, causing deterioration of the quality of copied images.
Further, the conventional mono-component development is implemented in the arrangement that a toner-layer doctor blade of an elastic body or the like is provided, spaced from the toner support member at a narrow gap or in slightly contact therewith, where a toner thin layer is formed on the toner support member. In this arrangement, it is likely that agglomerations of the residual toner bring about clogging in proximity to the toner-layer regulator blade, causing white voids. The residual toner may have silicon oxide buried into the toner by the pressure involved in the cleaning blade process, resulting in changes in the adhesion state of the silicon oxide or in defects or cracks of the toner. This accounts for deterioration of the fluidity of the residual toner. Such toner that is lowered in fluidity tends to bring about agglomeration. Those factors present difficulties recycling of toner.
The above-described situation results in not only the impossibility of effective utilization of resources, but may cause pollution of the environment as well. Consequently, it is a problem of great urgency to recycle the residual toner for re-utilization of resources, in terms of the environmental protection.